Antimicrobial resistance (AMR) is a significant threat to global health security and a one-health challenge because antibiotic use in both human and animal medicine contributes to the emergence, amplification, persistence and dissemination of AMR bacteria. Control of AMR currently relies primarily on prudent use principles and regulatory controls and on development of alternative antibiotics and infection control measures. However, limiting antibiotic use only works against existing AMR populations if there is a significant fitnes cost associated with carriage of AMR traits. In practice, this PASSIVE decay mechanism is largely ineffective because fitness costs for harboring resistance traits are low. This project wil develop a completely novel strategy to artificially induce a high fitness cost against AMR bacteria and ACTIVELY drive these microbes out of select populations. The focus will be tetracycline efflux pumps that are only expressed in the presence of a tetracycline antibiotic. It s possible to induce expression of tet(A) and tet(B) efflux pumps by exposing the bacteria to a degraded tetracycline that does not harm tetracycline-sensitive bacteria. A combination of expressing these efflux pumps in the presence of cofactors is known to impose a significant fitness cost on the tetracycline-resistant bacteria, but this idea has never been extended to population-level control. This project will test the central hypothesis that in the presence of specific cofactors, expression of tetracycline-resistance efflux pumps will impose a significant fitness cost on the host bacterium and this mechanism can be exploited to reduce the prevalence of antibiotic-resistant bacteria. With a series of lab-based and field-based experiments, this project seeks to (1) Determine if active selection can be generalized to additional tetracycline efflux pumps and to bacterial pathogens. This aim will identify cofactors, estimate the variance of the response by different tetracycline-resistant bacteria, and identify th component of the degraded tetracycline that is responsible for inducing expression of tetracycline efflux. (2) Determine if oral administration of a modified tetracycline product, with r without a cofactor, can reduce the prevalence of multidrug resistant E. coli in an animal model. This aim will employ a chicken model to determine if active selection can be used to limit AMR populations that could otherwise be disseminated to people. (3) Determine if addition of a modified tetracycline, with or without a cofactor, can reduce the prevalence of resistant E. coli found in soil reservoirs. Reservoirs of AMR bacteria likely play an important role in the long-term persistence of these bacteria in the environment. If we are able to selectively target primary reservoirs by active selection, this will reduce the overall prevalence of AMR bacteria in food chain and possibly in other environments including hospitals. Employing active selection against AMR bacteria is completely novel and innovative, and will result in rapid loss of AMR bacteria as compared to reliance on passive decline. It also has the advantage of leaving the antibiotic-susceptible population of bacteria largely unaffected and thus limit disruption of normal microbial flora.